Where have all the fliers gone?

Common insect-eating birds suffer dramatic declines…

New research by Bird Studies Canada (BirdLife Partner in Canada) has highlighted alarming trends in insect-eating birds. In the last two decades alone, populations of many common bug-eating species – such as Sand Martin Riparia riparia, Common Nighthawk Chordeiles minor and Chimney Swift Chaetura pelagica - have declined by over 70%. With many of the birds’ prey species also being important plant pollinators, the consequences may be felt by more than just birds.

The disturbing trend, which appears to be most pronounced in Canada, has been revealed by analysis of results from several North American bird monitoring programmes. There are many possible causes for these declines, including lack of nest sites, habitat change, pesticide usage and climatic variation.

However, according to the report’s author Jon McCracken, this tells only part of the story. “Potential changes in the seasonal supply of flying insects are worth investigating. If significant changes are occurring in insect populations, then this is a particularly serious cause for concern, not only for the birds and bats that feed on them, but also for us. There is now growing concern that pollinating-insects such as wasps, butterflies and moths are also declining, and that this may be fuelling the changes observed in birds which specialise upon them. Mr McCracken notes: “Since so many species of flying insects are important pollinators, there could be large ecological and socio-economic ramifications.”

Interestingly, preliminary examination suggests that birds which forage exclusively at heights less than about three metres tend to be faring much better. “Amongst the myriad of factors that may be working to reduce populations of aerial foragers, there is merit in thinking vertically as well as laterally” Mr McCracken added.

Without solid insights into the mechanisms that are driving population changes, it is difficult to imagine how the current decline of aerial insectivores can be halted, let alone reversed. However, one thing is clear - to come up with the answers, a massive research effort is now needed. “The magnitude and speed of the declines for some species is quite disturbing,” said Dr Phil Taylor (Chief Scientist, Bird Studies Canada). “It is vital to narrow down the possible reasons as soon as we can.”